1. Field of the invention.
The present invention relates to an apparatus for the application of a liquid or pasty medium to a moving base.
2. Description of the related art.
A base for the application of a liquid or pasty medium, in the case of direct application, is the surface of a fiber material web, in particular of paper or cardboard. Alternatively, in the case of indirect application, the surface of a transfer element, for example a transfer roll, transfers the medium to the material web, the medium being brought into contact with the moving base in an application chamber. The application chamber is bounded on the inlet side by a metering doctor device, with respect to the running direction of the base with the two doctor devices being arranged on a carrying beam.
An application apparatus of this type is disclosed, for example, by DE 44 44 779 A1. In the case of this application apparatus, the metering doctor device bounding the application chamber on the outlet side is designed as a roll doctor, whose doctor bed is fixed directly on the carrying beam via a holding blade. In order to influence the setting of the metering doctor against the base in accordance with the operation, an actuating device is provided. On the inlet side, the application chamber is likewise bounded by a roll doctor device, the doctor bed accommodating the doctor rod being fixed to a carrier beam by a spring-loaded holder. Specific influence of the setting of the sealing doctor rod against the base is not possible during the continuous operation of the application apparatus. Because of the above-described holding of the doctor devices by flexible blades, the distance between the two roll doctors can vary slightly, which entails a corresponding variation in the volume of the application chamber. Consequently, the medium has to be fed to the application chamber with a high excess in order to ensure in the application chamber pressure relationships which permit a reproducible coat weight of the medium leaving the application chamber of the application layer to be obtained.
The same is also true of the application apparatus disclosed by EP 0 319 503 B1 (see, in particular, FIG. 10) and the application apparatus disclosed by DE 84 14 904 U1. The two last-mentioned application apparatuses additionally have the disadvantage that the doctor bed or the blade holder of at least one of the two doctor devices is also used at the same time for bounding the feed line for feeding the medium into the application chamber. On the one hand, this creates sealing problems and, on the other hand, when the setting of the respective doctor device against the moving base is changed, it also inevitably leads to an influence being exerted on the quantity of medium fed to the application chamber per unit time (medium feed rate). Such an influence further has a detrimental effect on the pressure relationships prevailing in the application chamber.
In addition, DE 196 52 289 A1 discloses an application apparatus of the generic type in which both the sealing doctor device and the metering doctor device are a doctor rod. The two doctor rods are accommodated in a common doctor bed, in which the feed line for the medium is also located. The feed line is fixed to the carrying beam via an elastic blade. Furthermore, two actuating devices are provided, one of which acts on the doctor bed in the region of the sealing doctor rod and the other acts in the region of the metering doctor rod. Although, in the case of this application apparatus, on the one hand the size of the application chamber and on the other hand the quantity of medium fed per unit time depend to a lesser extend on the setting of the two doctor devices against the base. The disadvantage with this arrangement is that a change in the influence of one of the two actuating devices on the doctor bed always leads to an influence being exerted on the setting of the respective other doctor device against the moving base, so that coat weight and excess quantity cannot be adjusted independently of each other.
For completeness, reference should further be made to DE 29 13 053 A1 in relation to the prior art.
By contrast, the present invention specifies an application apparatus of the generic type which, while maintaining the capability of independently exerting an influence on the setting of the two doctor devices against the moving base, makes it possible to obtain defined volume and/or pressure relationships in the application chamber.
According to the invention, an application apparatus of the type mentioned at the beginning uses two doctor devices which are fixed to an essentially rigid supporting unit provided on the carrying beam. On account of the essentially rigid construction of the supporting unit, the doctor devices are kept at a predetermined distance from each other in the running direction of the base. On account of the rigidity~of the supporting unit, this predetermined distance is, in this case, essentially independent of the quantity of medium fed to the application chamber per unit time, the pressure prevailing in the application chamber and the respective setting of the two doctor devices against the base. Furthermore, the essentially rigid supporting unit ensures a defined arrangement of the two doctor devices in the transverse direction of the base as well. Overall, in this way, the conditions prevailing in the application chamber and medium feed, in particular volume and/or pressure, can be reproduced so reliably that the excess quantity which leaves the application chamber past the sealing doctor device can be reduced considerably, as compared with the prior art and, consequently, a high-quality application layer can be obtained. As a result of reducing the excess quantity, the circulated quantity of the medium can be reduced, and thus the delivery pump assigned to the medium feed can be designed to have a correspondingly lower performance and therefore to be more cost-effective in procurement and operation.
Moreover, the essentially rigid supporting unit permits the two doctor devices to be fixed to it in such a way that their settings against the moving base can be influenced independently of each other, without a change to the setting of one of the two doctor devices which would otherwise result in back-effects on the setting of the respective other doctor devices. The resulting decoupling of the two doctor devices simplifies their driving and thus increases their adjustment potential. This can be achieved, for example, by making it possible for the two doctor devices to be fixed to the supporting unit independently of each other.
In a development of the invention, it is proposed that the supporting unit include feed channels which feed the medium from a distribution pipe to the application chamber. In this way, the influence exerted on the quantity of medium fed to the application chamber per unit time can be decoupled largely, if not completely, from the influence exerted on the setting of the two doctor devices against the base. For example, the feed channels can be constructed as through holes, bores or the like in the supporting unit. Alternatively, the feed channels could also run through at least one of the doctor devices, for example through a doctor bed of a roll doctor device. In particular in the case of an application apparatus with a relatively low operating width and/or an application apparatus for applying medium at a relatively low coat weight, it may be advantageous for the distribution pipe also to be formed in the supporting unit. This simplifies, at least, the construction of the carrying beam.
In order to be able to achieve the most uniform possible distribution of the medium emerging from the feed channels in the transverse direction of the base, that is to say over the operating width, it is proposed that at least one baffle plate be arranged on the supporting unit in the area of the outlet openings of the feed channels into the application chamber. This baffle plate prevents the medium emerging from the feed channels from immediately contacting with the base, and thus ensures the deflection of the medium both in the running direction and in the transverse direction of the base. Here, the movement components in the transverse direction ensure uniform distribution of the medium over the operating width, while the movement components in the running direction prevent the production of transverse vortices which have a detrimental influence on the application result.
The distance between the baffle plate and the outlet openings and/or the shape of the baffle plate can be chosen on the basis of the operating conditions with regard to exerting a favorable influence on the resulting application. The distance between the baffle plate and the outlet openings can, in this case, be changed, for example, by fixing the baffle plate detachably to the supporting unit and interposing appropriate spacers.
A simple and nevertheless precise type of attachment of the doctor devices to the supporting unit can be provided, for example, by fixing the metering doctor device and/or the sealing doctor device to the supporting unit in a form-fitting manner.
For attachment to the supporting unit, at least one of the two doctor devices can be assigned at least one force appliance, which is preferably supported on the supporting unit or on a part connected thereto. Furthermore, at least one of the two doctor devices, preferably both thereof, can respectively be assigned at least one force appliance for exerting an influence on its setting against the moving base. This setting force appliance can also be supported on the supporting unit or on a part connected thereto.
The sealing doctor device mainly has the function of sealing off the application chamber against the ingress of air. That is to say, in particular, to keep away from the application chamber an air boundary layer carried along at the surface of the moving base. Thus, with regard to the simplest possible design of the control unit of the application apparatus according to the invention, it is sufficient for the setting of the sealing doctor device against the moving base to be capable of being adjusted without closed-loop control. On the other hand, since the metering doctor device is used to set the magnitude of the coat weight of the medium applied to the moving base, it is advantageous if the setting of the metering doctor device against the moving base can be controlled.
Both the at least one fixing force appliance and the at least one setting force appliance can be force appliances which can be operated hydraulically and/or pneumatically and/or hydropneumatically and/or electrically and/or magnetically and/or by electric motor and/or electrochemically.
With regard to the ability to change the transverse profile of the layer of medium applied to the base, it is further proposed that at least one of doctor devices, preferably at least the metering doctor device, be assigned a number of force appliances which are arranged adjacently in the transverse direction of the base and which can be driven and/or controlled independently of one another, and thus permit an influence to be exerted section by section on the setting of the doctor device against the base.
In order to prevent contamination of the force appliances and thus be able to ensure their serviceability, in a development of the invention it is proposed that a flat covering element be provided between at least one of the doctor devices and at least one of the force appliances assigned to this doctor device. The use of a covering element of this type is advantageous, in particular in the area of the sealing doctor device, since in order to avoid the ingress of air into the application chamber, it is quite common to feed the medium to the application chamber in excess and to permit excess medium to emerge from the application chamber past the sealing doctor device in the direction opposite to the running direction of the base.
In a manner known per se, the metering doctor device and/or the sealing doctor device may include a doctor rod. In this case, the metering doctor rod and/or the sealing doctor rod can be driven in rotation. In particular, a metering doctor rod rotating in the direction opposite to the running direction of the base can improve the quality of the resulting application.
In a development of the invention, it is proposed that the diameter of the metering doctor rod be alternately equal to or smaller than the diameter of the sealing doctor rod.
Furthermore, the two doctor rods can either have a smooth or a profiled surface; however, in order to achieve a good sealing action, it is advantageous if at least the sealing doctor rod has a profiled surface.
In principle, the supporting unit can be provided as an integral part of the carrying beam. If, however, the supporting unit is a unit constructed separately from the carrying beam, then the carrying beam ensures the stability of the application apparatus with regard to bending effects caused by gravity. Thus, the supporting unit itself does not need to perform any stabilization function and can therefore be constructed as a part which has a relatively light weight and therefore be simple to handle. The supporting units can be fixed to the carrying beam at a relatively low number of fixing points, which simplifies the machining of the carrying beam.
Furthermore, the low number of fixing points of the supporting unit on the carrying beam permits the rapid changing of the supporting unit and the doctor devices arranged on it. Thus, during the continuous operation of the application apparatus, using a first supporting unit with doctor devices arranged thereon, a second supporting unit with unworn doctor devices can be readied at an equipment workplace and then, when the application apparatus is at a standstill, can be substituted relatively quickly and simply for the first supporting unit. Using such a changeover procedure, the unproductive times of the application apparatus can be reduced.
If the loading of the sealing doctor device can be controlled indirectly or directly on the basis of the pressure prevailing in the application chamber, this helps to prevent the production of an undesired excess of pressure in the application chamber. Furthermore, it is possible to influence the coat weight of the medium leaving the application chamber as an application layer by the loading of the metering doctor device and sealing doctor device being varied relative to each other.
In addition or as an alternative, it is moreover possible for the metering doctor device and/or the sealing doctor device to be a doctor blade.